The long-term aim of this proposal is to investigate the effect of rigidity of fixation on early fracture healing. Preliminary studies indicate that the stiffness of fracture fixation determines the balance between new bone formation and bone resorption. With a less firmly fixed fracture, new bone formation is less and bone porosity (bone resorption) is greater than the paired, more firmly fixed fracture. The end result is that biomechanically the quality of fracture healing at 120 days is less on the less firmly fixed side as judged by maximum torque, torsional stiffness and energy absorption. A further aim is to develop a non-invasive measurement of bone blood flow. Looking to the future, it might be possible to measure biologic response by blood flow determinations and measure fracture rigidity by an instrumented external fixator. Initially single plane versus bi-planar fixation will be compared by using a newly designed canine external fixator. This fixator will be applied to bilateral standard tibial osteotomies in the same animal. In the same manner, and oblique fracture (osteotomy 30 degrees from the long axis of the bone) will be fixed with an external fixator on one side, and with an external fixator and screw fixation on the opposite side. Separate studies will be performed to anlayze bone response about pin-tract sites under high pin bending moment. The basic hypothesis is that bone resorption and remodeling due to localized high stresses at the pin-bone interface is a possible cause for ping loosening and sepsis in external fixation of fractures. The rationale is that the response to biomechanical effects on bone can be tested by in vivo experiments that quantitate the biologic and results in terms of localized measurement of bone formation and bone resorption. Blood flow can be measured in bone by microsphere methods. It is our plan to test a hypothesis that early distribution and uptake (10 - 60 sec) of bone concentrating tracers will be directly related to bone blood flow. If this hypothesis is true, then a non-invasive method ot estimate blood flow during fracture healing offers the possibility of quantitating the healing process during the early phases of fracture fixation. In these experiments, it is planned to determine the ideal mechanical environment for fracture healing utilizing external fixation techniques and to test these observations with physiological parameters, measurement of blood flow and bone remodeling (formation of new bone and resorption of existing bone).